1. Field of the Invention
The present invention relates to an electrical contacting device for electrically contacting a fuel cell, and particularly for contacting carbon separator plates in a solid polymer electrolyte fuel cell stack.
2. Description of the Related Art
Electrochemical fuel cells convert a fuel and oxidant to generate electrical power and reaction products. A preferred type of fuel cell is the solid polymer electrochemical fuel cell which employs a solid polymer electrolyte or ion exchange membrane. The membrane electrolyte is generally disposed between two electrode layers (a cathode and an anode layer) to form a membrane electrode assembly (“MEA”). In a typical solid polymer electrolyte fuel cell, the MEA is disposed between two electrically conductive separator or fluid flow field plates. Fluid flow field plates have at least one flow passage formed therein to direct a fluid reactant (either fuel or oxidant) to the appropriate electrode layer, namely the anode on the fuel side and the cathode on the oxidant side. The separator or flow field plates also act as current collectors and provide mechanical support for the MEAs.
Since the output voltage of a single fuel cell is relatively low (e.g. less than a volt), fuel cell power supplies typically contain many cells that are connected together, usually in series but sometimes in parallel, in order to increase the overall output voltage and power of the supply. In a series configuration, the fuel cells are typically arranged in a stack such that one side of a given separator plate serves as an anode side plate for one cell while the other side of the plate serves as the cathode side plate for the adjacent cell. Such a separator plate is referred to as a bipolar plate. A stack of multiple fuel cells is referred to as a fuel cell stack. The fuel cell stack is typically held together in its assembled state by tie rods and end plates. A compression mechanism is generally required to ensure sealing around internal stack manifolds and flow fields, and also to ensure adequate electrical contact between the surfaces of the plates and MEAs.
The bipolar plates in these fuel cells must meet certain mechanical, electrical, and corrosion resistance requirements. Metals may be considered for use in plate constructions, but many common metals and alloys are not suitable due to inadequate corrosion resistance. While corrosion resistant metallic compositions may instead be considered, difficulties are frequently encountered in making electrical contact through the passivating surface layers of these compositions. Coatings of various sorts have been proposed to allow for the use of metallic bipolar plates. For instance, as disclosed in published European patent application EP 1107340, an electrically conductive corrosion resistant polymer containing electrically conductive corrosion resistant filler particles may be used to coat the working faces of bipolar plates. A preferred alternative to metallic compositions is to use a suitable carbon for plate construction since carbon plates can be made suitably conductive and exhibit good corrosion resistance.
To draw power from the fuel cell stack, low resistance electrical connections are typically provided at each end of the fuel cell stack using a pair of copper or coated copper bus plates. It may, however, be desirable to electrically connect to one or more electrodes in the fuel cell stack for other reasons. These other electrical connections are typically not intended to carry the entire stack current. For instance, it can be useful to monitor individual cell voltages to detect for abnormally low voltages during operation. In turn, corrective action can then be taken to prevent a cell or cells from undergoing voltage reversal, and thus to prevent reversal-related damage from occurring to the cell and/or stack. (Voltage reversal can occur in a weaker cell in a series stack when that cell is incapable of providing current at the same level as other cells in the stack. In such a situation, a sufficiently high current generated by the other cells in the stack is forced through the weaker cell and drives it into voltage reversal.) Measuring each cell voltage and individually comparing each voltage to a reference voltage may seem onerous in practice. However, simple circuitry may be employed to detect low voltage conditions on a cell or cells and then to signal for corrective action.
Making reliable electrical connections to individual cells in such a fuel cell stack can be problematic though, particularly to cells employing carbon separator plates. As designs of fuel cells advance, the separator plates have become progressively thinner and more closely spaced. This makes it more difficult to align and install electrical contacts to the plurality of fuel cells in a stack. Further, the cell-to-cell spacing (i.e., cell pitch) is subject to variations due to manufacturing tolerances and to expansion and contraction during operation of the stack (as a result of thermal variations, internal pressure changes, and gradual compression of cell components over time). Thus, suitable connections must accommodate these variations. Further still, the fuel cell stack may be subject to vibration and thus reliable connections must be able to maintain contact even when subjected to vibration. Inappropriately installed connectors may also interfere with seals in the fuel cell.
Additional problems arise when employing conventional metal compositions for the connectors. In the immediate vicinity of a fuel cell, the environment may be humid, hot, and either acidic or alkaline. For example, in solid polymer electrolyte fuel cells, carbon separator plates may be somewhat porous. The environment in the immediate vicinity of the plates can therefore be somewhat similar to that inside the cells, with the consequence that the metallic connectors may be subject to corrosion. In turn, the connector may also be a source of contaminants. Further, the relatively good electrical conductivity of metallic connectors can be a disadvantage in the event of an inadvertent short between connectors that are connected to different cells in a series stack. Large currents can flow through such an inadvertent short thereby representing a hazard.
Various contacting devices have been considered for making such electrical connections. Copper tabs and spade type connectors have been contemplated but exhibit many of the aforementioned disadvantages. Published PCT patent application WO99/66339 for example shows a device employing flexible spring wire contacts that make a pressure connection to components in a fuel cell stack. Published European patent application EP1001666 shows the use of a flexible printed circuit board for making electrical connections to components in a fuel cell stack.
Accordingly, there remains a need for improved electrical contact within a fuel cell, particularly for contacting carbon separator plates in a solid polymer electrolyte fuel cell stack. This invention fulfills these needs and provides further related advantages.